This application is based on and claims the benefit of German Patent Application No. 198 22 616.0 filed May 20, 1998, which is incorporated by reference herein.
This invention relates to a light source and to a method for the transmission of digital, spectrally coded data as described in the introductory portion of claim 1.
The prior art, e.g. German Patent Application 197 231 03.9, discloses transmission methods that utilize spectral coding. Such an optical transmission network consists of optical transmission lines and optical splitters and may also include optical amplifiers, and is used to transmit coded, multiplexed optical signals. Each transmitter contains a coder in which the signals to be transmitted are coded before they are transmitted into the optical transmission network. The coding is done optically, e.g. by frequency coding with an optical filter. Each receiver that wants to receive the data from a specific transmitter, must contain a decoded that is tuned to the coder of this specific transmitter. In the simplest case, the frequency ranges that pass optical signals and the frequency ranges that block optical signals are the same in the coder and in the decoder. This decoding method is known by the term CDMA (Code Division Multiple Access). In these systems, light-emitting diodes are used, the broadband emission spectrum of which passes through an optical filter. The optical filter can, for example, be a Fabry-Perot filter which converts the broadband spectrum into a frequency comb. A significant disadvantage of these systems of the prior art is the low optical output of the broadband light sources that are available. The light-emitting diodes currently used emit around 1,550 nm, and even then with a very low output ( less than 0.1 milliwatt), which in connection with passive optical filters leads to transmission outputs in the range of approximately 30 xcexcW. Such systems therefore always require an optical amplifier.
The light source claimed by the invention, with the characterizing features disclosed in the independent claims, has the advantage over similar systems of the prior art that significantly higher outputs can be achieved, which therefore makes it possible to do without an optical amplifier. The broadband frequency comb, which is necessary for the use of the CDMA method, is actively created. Thus the injection current of the solid state laser within a bit period can be frequency-modulated or phase-modulated in the shape of a sine-wave.
Advantageous refinements and improvements to the light sources disclosed in the independent claims can be achieved by means of the measures disclosed in the subclaims. It is particularly advantageous that the spectral width of the frequency periodic output of the solid state laser is determined by the ratio of the frequency shift and the modulation frequency, and is therefore adjustable.
A DFB (Distributed Feedback Laser) is advantageously used. A Fabry-Perot laser can also be used as the light source for the CDMA method.